Uniform mesoporous 3D hematite superstructures: phase transition and their magnetic properties

Uniform mesoporous superparamagnetic 3D hematite superstructures self-assembled using nanoparticles as building blocks have been successfully obtained in large quantities via an oleic acid-assisted one-pot solvothermal route. The as-obtained products were characterized by X-ray diffraction, X-ray photoelectron spectroscopy, Raman spectroscopy, scanning electron microscopy, transmission electron microscopy, Fourier transform infrared spectroscopy and N-2 absorption-desorption isotherm. Some factors influencing the phase and morphology of the products were systematically investigated. Reaction time plays an important role in phase transition of the products from magnetite (microspheres), to hematite (mesoporous 3D superstructures), to maghemite (magnetic chains). Notably, ultrafine nanoparticles or cubic superstructures can also be obtained without oleic acid or instead by other additives. Oleic acid-assisted self-assembly and dissolution-recrystallization process is proposed based on experimental investigation and analysis. The magnetic study reveals a surprising phenomenon that the mesoporous 3D hematite superstructures exhibit a unique superparamagnetic behavior with a high saturation magnetization at room temperature, which is seldom reported thus far for the mesoporous 3D hematite superstructures. Uniform mesoporous superparamagnetic 3D hematite superstructures probably have potential application in magnetorheology, catalysis and water treatment.